A frequency selective surface (FSS) denotes a plane or a surface that is formed of regularly arranged patterns each having a predetermined shape in order to have a frequency selective characteristic. The FSS has characteristics of passing or stopping a predetermined frequency band according to a geometric structure such as a shape, a size, a length, and a width of the patterns and according to the electric characteristics of dielectric. In the FSS, each of the regularly arranged patterns, a spatial single cycle, is a unit cell. The frequency characteristic of the FSS significantly changes according to the shape of a pattern in a unit cell, a geometric structure, a size, a distance between unit cells, and the electric characteristics of a dielectric. There have been many studies in progress for developing various methods for obtaining desired frequency characteristics.
Conventionally, various structures such as a center connected rod shaped structure and a loop structure have been introduced as a unit cell in a FSS structure for filtering a predetermined frequency band. In order to design a FSS to have the maximum length in a unit area, a shape of a loop must be compound bent and not entangled. Therefore, many FSS structures that maximally use the space of a unit cell have been introduced in order to improve the spatial utilization. For example, one of representative conventional FSS structures is a Hilbert curve using a fractal curve.
A first conventional FSS technology was introduced in U.S. Pat. No. 5,384,575 entitled Bandpass frequency selective surface issued at Jan. 24, 1995. The first conventional FSS technology relates to embody a band-pass filter using a FSS for passing a predetermined frequency band. In the first conventional FSS technology, the resonant frequency of a FSS can be controlled by controlling the width and the overall length of the dielectric slot.
The first conventional FSS technology, however, has disadvantages as follows. The first conventional technology taught only about a FSS structure for passing a predetermined frequency band. The first conventional FSS technology uses a rectangle conductive loop in a unit cell for embodying a band-pass filter and controls the overall length of the rectangle conductive loop to fine tune the resonant frequency of the FSS. If the length of the loop is controlled to fine tune the resonant frequency, the area of the unit cell also changes.
A second conventional FSS technology for designing a FSS resonating at a desired frequency band while reducing the area of a unit cell was introduced in an article entitled Convoluted array elements and reduced size unit cells for frequency-selective surface by E. A. Parker and A. N. A. EI sheick, IEEE PROCEEDINGS-H, Vol. 1 vol. 138, no. 1, February 1991, pp 19-22. In the second conventional FSS technology, Hilbert curve is used to form a unit cell. The second conventional FSS technology also has disadvantages as follows. The second conventional FSS technology uses a convoluted conductive square to form a unit cell, and the resonant frequency changes according to input polarization such as vertical polarization or horizontal polarization.
In order to overcome the problems of conventional FSS technologies, another conventional FSS structure, hereinafter, a FSS structure of 41180, was introduced in Korea Patent Application 2005-41180 filed at May 17, 2005 by the applicant of the present invention. The conventional FSS structure of 41180 has a rectangle meander loop which is bent at least one or more times, and the length of the rectangle meander loop is controlled to obtain a desired filtering characteristic.
The conventional FSS structure of 41180, however, cannot be used in a part required to transmit light because the transmittance of light is degraded by the meanderingly bent rectangle loop without entangled each other.